I got the Paladin sidequest, off of Trick Totem as Mage on turn 2, that gives all your minions +1/+1 when you summon 5 minions.
On a later turn, I played Firebrand. My board had been wiped before that, I played Firebrand on an empty board with the intention of using his spellburst to kill the enemy 3/2. When the sidequest triggered, Firebrand hit the board buffed, but lost his Spellburst.
That’s definitely a bug, because Spellburst should only trigger when we play a spell card, not when the result of a sidequest resolves.